Transparent organic solar cells could contribute to greener buildings

A new design for a semi-transparent plastic solar cell may be an important step toward the advance of building-integrated photovoltaics (Photo: ICFO)

Researchers at the non-profit Institute of Photonic Sciences (ICFO) have created a new design for an organic solar cell that retains good efficiency while being flexible, thin, and almost completely transparent. Put together, these characteristics may make the cells an ideal candidate for building-integrated photovoltaics.

Organic solar cells (OPVs), or plastic solar cells, are a type of cell that employs organic conductive polymers to collect energy from the Sun. While their efficiency and cost per watt doesn't quite measure up to that of your standard silicon cell, plastic cells come with their own unique perks – among other things, they are lighter, more flexible, and better suited to absorbing indirect sunlight.

OPVs achieve maximum efficiency when they are opaque, but scientists can also choose to sacrifice efficiency to make them semitransparent by thinning the back metal electrode down to just a few nanometers. While this can be useful for some applications, such as solar windows, cell efficiency usually takes a serious hit in the process.

Now, researchers at ICFO have announced that they were able to build an organic solar cell with a transparency that they claim is practically indistinguishable from that of normal glass, and is still nearly as efficient as an opaque plastic cell.

The scientists did so by incorporating a photonic crystal inside the panel, which increased the amount of infrared and ultraviolet light absorbed by the cell. Though far from mind-blowing, the transparent cells' efficiency was measured at 5.6 percent, which is about half of the record efficiency for solar cells of this type.

The cells developed at ICFO hold promise for building-integrated photovoltaics. They offer few constraints, as they can be employed on a curved surface and, though they are nearly transparent, their color can also be tuned by changing the configuration of the photonic crystal.

The researchers are now working on boosting the capabilities of the cells, improving their stability, lifetime, and attempting to substantially raise their efficiency.

"In the midterm we expect to reach the extremely high transparencies and efficiencies needed to power up devices such as displays, tablets [and] smart phones," says Prof. Jordi Martorell, who led the study.